Discharge lamp with dielectrically impeded electrodes

ABSTRACT

A discharge lamp, suitable for operation by means of dielectrically impeded discharge, having electrodes arranged on the wall of the discharge vessel, has at least one dielectric layer which covers at least a part of the electrodes and, optionally, the discharge vessel wall as well. A phosphor and/or reflective layer is arranged on the at least one dielectric layer. According to the invention, at least the dielectric layer arranged directly underneath the phosphor or reflective layer consists of a glass solder whose viscosity variation as a function of temperature is irreversible, in particular of a sintered glass ceramic. This prevents this layer from re-melting during the fabrication process and thereby tearing the overlying porous reflective and/or phosphor layers.

TECHNICAL FIELD

The invention relates to a discharge lamp according to theprecharacterizing clause of claim 1.

The term “discharge lamp” here covers sources of electromagneticradiation based on gas discharges. The spectrum of the radiation can inthis case cover both the visible range and the UV (ultraviolet)/VUV(vacuum ultraviolet) range, as well as the IR (infrared) range.Furthermore, a phosphor layer may also be provided for convertinginvisible radiation into visible radiation.

The case in point deals with discharge lamps having so-calleddielectrically impededed electrodes. The dielectrically impededelectrodes are typically produced in the form of thin metal strips, atleast a part of which is arranged on the inner wall of the dischargevessel. At least a part of these inner-wall electrodes is fullyconcealed from the interior of the discharge vessel by a dielectricbarrier layer.

If only electrodes of a single polarity—preferably the anodes—arecovered with a dielectric barrier layer, then in preferable unipolaroperation a so-called unilaterally dielectrically impeded discharge isformed. However, if all the electrodes, i.e. both polarities, arecovered with a dielectric barrier layer, then both in unipolar andbipolar operation a bilaterally dielectrically impeded discharge isformed.

On the dielectric barrier layer, and in general on all other parts ofthe inner wall of the discharge vessel as well, at least one otherfunctional layer is applied, e.g. a layer of a phosphor or phosphorblend and/or one or more layers which reflect visible radiation (light)and/or UV radiation. The purpose of the reflective layer is to send outvisible light in a controlled way, i.e. only in a particular preferreddirection of the lamp.

There are no particular restrictions on the geometrical shape of thedischarge vessel. For example, tubular or flat discharge vessels arecommonplace, the latter being amongst other things suitable as so-calledflat lamps for the back-lighting of liquid crystal displays (LCDs).

PRIOR ART

The starting materials for both the reflective and the phosphor layer orlayers are initially in the form of powders with a suitable grain size.These powders are then applied as a suspension, usually mixed with anorganic binder, with a defined layer thickness to the inner wall of thelamp or to the previously applied other functional layers, e.g.electrodes and dielectric barrier layer. The thickness of the reflectiveor phosphor layer is, controlled through the viscosity of thesuspension, adapted to the respective coating process. After drying andheating, the reflective and/or phosphor layers are in the form of porouspowder layer or layers.

Besides the phosphor layer thickness, the uniformity of the reflectiveand/or phosphor layer as well as its mechanical bonding strength, whichdecreases as the layer thickness increases, are also importantconditions for obtaining optimum conversion of UV light to visiblelight.

The dielectric barrier layer usually consists of glass frits, preferablylead borosilicate glass (Pb—B—Si—O).

In the case of flat lamps, whose discharge vessels respectively consistof an essentially plane base glass, a similar front glass and,optionally, a frame, the base glass is provided with a so-called solderedge which likewise consists of a glass frit, preferably PbB—Si—O. Thepurpose of this solder edge is to bond the components of the dischargevessel (base glass, frame, front glass) in vacuum-tight fashion duringthe assembly process. This assembly process involves carrying out athermal treatment in which the solder edge “melts” to a defined degree,i.e. reaches a defined viscosity.

The reflective and/or phosphor layers are usually applied before thisassembly process. Because of this, in addition to the solder edge, thedielectric barrier layer also returns to lower viscosity at the assemblytemperature. The overlying porous reflective and/or phosphor layers arehence in turn torn by the “movement” in the dielectric barrier layer(“ice-floe formation”). The reason for this is that the porous layershave no cohesion and hence cannot join in with this movement withoutdamage, but instead tear and/or even sink partly into the dielectricbarrier layer. The uniformity of the reflective and phosphor layer ishence compromised, which causes light losses. Furthermore, these “icefloes” are clearly identifiable during lamp operation as light-densitynon-uniformity, for example on the luminous side of a flat lamp.

DESCRIPTION OF THE INVENTION

The object of the present invention is to avoid the disadvantagesmentioned above and to provide a discharge lamp according to theprecharacterizing cause of claim 1 which has a phosphor and/orreflective layer improved in terms of homogeneity.

This object is achieved by the characterizing features of claim 1.Particularly advantageous refinements are described in the dependentclaims.

According to the invention, that layer which is arranged essentiallydirectly underneath the phosphor or reflective layer of the dischargelamp consists of a glass solder whose viscosity variation as a functionof temperature is irreversible. This feature is described in more detailbelow. For the sake of simplicity, this layer will also be referred tobelow as the “supporting” layer or “anti-ice-floe layer”.

In this context, essentially directly underneath the phosphor orreflective layer of the discharge lamp means that as far as possiblethere should be no other layer between the “supporting” layer and theporous phosphor or reflective layer, or at most only a very thin one.The maximum allowable thickness for such an additional layer is dictatedby the condition that, when the lamp is heated (heating up, assemblyprocess etc.) the porous phosphor or reflective layer arranged directlyabove must not be able to tear as a result of excessive “movement”because of the softening of the additional layer. Depending on itsmake-up and composition, the thickness of any additional layer shouldnot exceed 100 μm, preferably 50 μm, typically 10 μm, ideally 5 μm. The“supporting” layer is, however, preferably arranged directly underneaththe phosphor or reflective layer, i.e. without any additional layerbetween the “supporting” layer and the phosphor or reflective layer.

This “supporting” layer (“anti-ice-floe layer”) may be formed either bythe actual barrier layer acting as a dielectric impediment for thedischarge, or by an interlayer arranged between the dielectric barrierlayer, on the one hand, and the reflective and/or phosphor layer, on theother.

This interlayer should cover at least all of the dielectric barrierlayer, and may even be applied “full-surface”. For the effect accordingto the invention, it has been found to be sufficient if the thickness ofthis “supporting” interlayer is of the order of about 10 μm or more. Thesystem, typically in paste form, is applied using standard methods suchas spraying, dispensing, roller application, screen or stencil printing,etc.

The dielectric barrier layer can be applied both in strip form to theindividual electrodes (for unilateral and bilateral dielectricimpediment) and—in the case of bilaterally dielectrically impededdischarge—“full-surface” by means of a single continuous barrier layerwhich covers all of the inner-wall electrodes. The selection of thesuitable thickness for the barrier layer is essentially dictated byphysical discharge requirements and is typically of the order of 10 μmto several hundred μm, in particular between 50 μm and 200 the μm,typically between 80 μm and 180 μm. Furthermore—in the case ofbilaterally dielectrically impeded discharge—the thickness of thebarrier layer(s) for the anodes or cathodes may also be chosen to bedifferent. Preferably, in unipolar pulse operation (W094/23442), thebarrier layer for the anodes is thicker than that for the cathodes,although the layer thicknesses may also be equal.

The advantage of the first solution, i.e. the dielectric barrier layeris at the same time designed as the “supporting” layer (“anti-ice-floelayer”), is essentially that no additional fabrication or printing stepis necessary. On the other hand, the solution with the additionalinterlayer gives an additional degree of freedom for rational materialselection for the dielectric barrier layer, especially in terms of thedischarge-affecting dielectric as well as electrical properties.

For clearer understanding of the invention, the behaviour of the glasssolders customarily used as a supporting glass layer for the porouslayers will be explained first. Normally, hence also in the case of thePb—B—Si—O glasses, the viscosity decreases as the temperature increases.This behaviour is reproducible as long as the temperature has not beenso high that devitrification has already taken place. The termreproducible means that the temperature range in which the glass softenswith defined viscosity is virtually constant even under repetition, i.e.in each case after corresponding prior cooling.

Conversely, the glass solders proposed according to the invention do notexhibit this behaviour. Instead, their viscosity variation as a functionof temperature is irreversible. In this case, the viscosity does in factdecrease initially as the temperature increases.

Subsequently, however—even with further increasing temperature—anincrease in viscosity once more takes place.

This variation in viscosity as a function of temperature is actuallyexhibited, in particular, by per se known crystallizing glass solders,the use of which as a layer arranged directly underneath the phosphor orreflective layer of the discharge lamp is proposed according to theinvention. The aforementioned viscosity increase at constant or evenincreasing temperature is caused in crystallizing glass solders by theonset of the crystallization process. Using a defined temperatureprofile, the crystal growth as well as the phase composition and thecrystallite size can also be controlled. The so-called sintered glassceramic obtained in this way is distinguished in that, during asubsequent thermal treatment, it does not start to soften until highertemperatures, typically temperatures about 50-100° C. or more higher.

This meets the requirement of obtaining a “supportive” layer which issolid at the assembly temperature, i.e. more highly viscous, on whichthe porous layers can be printed. Through the use of such sintered glassceramic layers, continuous reflective and/or phosphor layers areobtained, in particular after the assembly process. Bismuth borosilicateglass (Bi—B—Si—O) has proved to be a particularly suitable crystallizingglass solder. Examples of other suitable crystallizing glass soldersinclude zinc bismuth borosilicate glass (Zn—Bi—B—Si—O) and zincborosilicate glass (Zn—B—Si—O).

Good results have also been obtained with certain composite solders withsimilar viscosity/temperature behaviour.

DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail below with reference toseveral illustrative embodiments.

FIG. 1a shows a schematic representation of a partly cut-away plan viewof a flat discharge lamp according to the invention with electrodesarranged on the baseplate,

FIG. 1b shows a schematic representation of a side view of the flat lampin FIG. 1a,

FIG. 1c shows a partial sectional representation of the flat lamp inFIG. 1a along the line I—I, and

FIG. 2 shows a partial sectional representation of a variant of the flatlamp in FIG. 1a along the line I—I.

FIGS. 1a, 1 b and 1 c respectively show, in schematic representation, aplan view, a side view and a partial section along the line I—I of aflat phosphor lamp, which emits white light during operation. It isdesigned as back-lighting for an LCD (Liquid Crystal Display).

The flat lamp 1 consists of a flat discharge vessel 2 with rectangularbase surface, four strip-like metal cathodes 3, 4 (−) and anodes (+), ofwhich three are designed as elongate double anodes 5 and two as singlestrip-like anodes 6. For its part, the discharge vessel 2 consists of abaseplate 7, a front plate 8 and a frame 9. The baseplate 7 and frontplate 8 are respectively bonded hermetically to the frame 9 by means ofthe glass solder 10 so that the interior 11 of the discharge vessel 2 isof cuboid form. The baseplate 7 is larger than the front plate 8 so thatthe discharge vessel 2 has a free edge running around it. The cut-out inthe front plate 8 serves only for illustration and gives a view of apart of the cathodes 3, 4 and anodes 5,6.

The cathodes 3, 4 and anodes 5,6 are arranged alternately and parallelon the inner wall of the baseplate 7. The anodes 6,5 and cathodes 3,4are respectively extended at one of their ends and are fed out of theinterior 11 of the discharge vessel 2 on both sides on the baseplate 7.On the edge of the baseplate 7, the electrode strips 3,4,5,6 each jointhe respective cathode-side 13 or anode-side 14 bus-like outerelectricity supply. The two outer electricity supplies 13, 14 are usedas contacts for connection to an electrical power source (not shown).

In the interior 11 of the discharge vessel 2, the electrodes 3-6 arefully covered with a sintered glass ceramic layer 61 of Bi—B—Si—O (cf.FIG. 1c), whose thickness is about 250 μm. On the one hand, this layercounteracts the “ice-floe formation”. On the other, the sintered glassceramic layer 61 acts at the same time as a dielectric barrier layer forall the electrodes 3-6. This is hence a case of bilateral dielectricimpediment. A reflective layer 62 of TiO₂, whose thickness is about 4μm, is applied on the sintered glass ceramic layer 61. On the reflectivelayer 62 in turn, and on the inner wall of the front plate 8, a phosphorblend layer 63 is applied (the layers are not represented in FIG. 1a forthe sake of clarity; cf. FIG. 1c), which converts the UV/VUV radiationproduced by the discharge to visible white light. This is a three-bandphosphor with the blue component BAM (BaMgAl₁₀O₁₇:Eu²⁺), the greencomponent LAP (LaPO₄:[Tb³⁺,Ce³⁺]) and the red component YOB([Y,Gd]BO₃:Eu³⁺). The thickness of the phosphor blend layer 63 is about30 μm.

The electrodes 3-6, including feed-throughs and outer electricitysupplies 13, 14, are respectively designed as a continuous cathode-sideor anode-side conductor-track layer-like structure. These two layer-likestructures, as well as the other functional layers whichfollow—dielectric barrier layer 61, reflective layer 62 and phosphorlayer 63—are applied directly on the baseplate 7 and front plate 8 bymeans of a screen printing technique.

After the layers 61-63 have been applied, the baseplate 7 is fused tothe frame 9, and the latter is in turn fused to the front plate 8, ineach case by means of glass solder 10, to form the complete flat lamp 1.The assembly process is carried out, for example, in a vacuum oven.Before the components of the discharge vessel are fused together, theinterior 11 of the flat lamp 1 is filled with xenon at a fillingpressure of 10 kPa. The two anode strips 5 a, 5 b of each anode pair 5are widened in the direction of the two edges 15, 16 of the flat lamp 1,which are oriented perpendicular to the electrode strips 3-6, and to beprecise asymmetrically exclusively in the direction of the respectivepartner strips 5 b and 5 a, respectively. The maximum distance betweenthe two strips of each anode pair 5 is about 4 mm, and the smallestdistance is about 3 mm. The two individual anode strips 6 are eacharranged immediately next to the two edges 17, 18 of the flat lamp 1which are parallel to the electrode strips 3-6.

The cathode strips 3; 4 have nose-like semicircular projections 19facing the respective adjacent anode 5; 6. These cause locally limitedamplifications of the electric field and consequently cause thedelta-shaped individual discharges (not shown in FIG. 1a) created inoperation according to WO94/23442 to be struck exclusively at thesepoints. The distance between the projections 19 and the respectivedirectly adjacent anode strip is about 6 mm. The radius of thesemicircular projections 19 is about 2 mm.

FIG. 2 shows a partial sectional representation of a variant of the flatlamp in FIG. 1a along the line I—I. The same features are given the samereference numbers. In contrast to the representation in FIG. 1c, anadditional 12 μm thick interlayer 64 of Bi—B—Si—O is in this casearranged between the dielectric barrier layer 61′ and the reflectivelayer 62. The dielectric barrier layer 61′ consists here of leadborosilicate glass. The function of the crystallizing layer, whichprevents the “ice-floe formation”, is hence undertaken here by theinterlayer 64.

In one variant (not shown), another reflective layer of Al₂O₃ isarranged between the TiO₂ layer and the phosphor layer. The reflectingeffect is improved in this way.

The thickness of the Al₂O₃ layer is about 5 μm.

In the scope of the invention, yet further additional layers and layerarrangements are conceivable, without the advantageous effect of theinvention being lost. All that is essential here is for that dielectriclayer whose viscosity variation as a function of temperature isirreversible and hence prevents the “ice-flow formation” to be arrangeddirectly underneath the phosphor or reflective layer (“supporting”layer).

At this point, it should again be pointed out that the layersrepresented very schematically in FIGS. 1c and 2 need not necessarily beextended over the entire surface of the baseplate. All that is essentialis for at least the relevant electrode to be fully covered with thecorresponding layers in each case. In the case of unilateral dielectricimpediment, only the electrodes of one polarity, preferably the anodes,are covered with a “supporting” dielectric layer.

Furthermore, the individual layers need not necessarily be entirelyplane, as represented in FIGS. 1c and 2 in a simplified manner. Instead,the individual layers, and in particular the very thin layers, may inpractice also be inherently uneven. This is found especially when one ormore layers are thinner than the electrodes and the layer(s) hence stillrecognizably reproduce the surface shape of the baseplate with theelectrodes.

Another illustrative embodiment (not shown) involves a tubular aperturelamp. Apart from the different shape of the discharge vessel, the maindifference from the flat lamp in FIG. 1 consists in the productionprocess tailored to the modified vessel shape. In particular, thephosphor is in this case applied to the inner wall, or the functionallayers previously arranged thereon, by applying a slurry. The principalsequence and function of the individual functional layers, in particularthe inventive effect of the “supporting” layer which prevents the“ice-floe formation”, correspond to those in FIG. 1.

What is claimed is:
 1. Discharge lamp (1), suitable for operation bymeans of dielectrically impeded discharge, having a discharge vessel (2)at least partially consisting of an electrically non-conductivematerial, electrodes (3-6) which are arranged on the wall (7) of thedischarge vessel (2), at least one dielectric layer (61; 64) whichcovers at least a part of the electrodes (3-6) and, optionally, thedischarge vessel wall (7) as well, a phosphor (63) and/or reflectivelayer (62) which covers the at least one dielectric layer (61; 61, 64),characterized in that at least the dielectric layer (61; 64) arrangedessentially directly underneath the phosphor or reflective layer (62)consists of a glass solder whose viscosity variation as a function oftemperature is irreversible.
 2. Discharge lamp according to claim 1, thesoftening temperature of the glass solder (61; 64) under repeatedheating being more than about 25°C. higher than the softeningtemperature of the glass solder in the first melting process. 3.Discharge lamp according to claim 1 or 2, the glass solder (61; 64)consisting of a crystallizing glass solder (sintered glass ceramic). 4.Discharge lamp according to claim 3, the sintered glass ceramic (61; 64)consisting of Bi—B—Si—O.
 5. Discharge lamp according to claim 1 or 2,the glass solder (61; 64) consisting of a composite glass solder.